You may not feel happy when you smell your husband’s underarm when he has not showered or used deodorant for 24 hours, but your dog probably does. So concluded scientists who conducted an fMRI study to investigate the response of dogs’ brains to both familiar and unfamiliar canine and human odors. Since the canine sense of smell is so well-developed, studies that investigate it are especially useful for learning more about dogs, including their behavior and emotions.

In this experiment, researchers focused on the caudate, which is an area of the brain that is associated with positive feelings and rewards. The level of activity in this part of the brain in response to various odors informs us about the emotional reaction of dogs to various stimuli. The odors used were the dog’s own odor, a familiar dog, an unfamiliar dog, a familiar person and an unfamiliar person. The familiar person was never the guardian handling the dog at the experiment because the scent of that person was present throughout the experiment.

The scientists found that dogs had the strongest, most positive reactions to the smell of a familiar person. Because most of the handlers with the dog during the experiment were female guardians, the familiar person was usually the male guardian or their child, although it was sometimes a close friend. The familiar dog was also a member of the household. The scents from dogs came from the perineal-genital area.

The dogs responded to all of the scents, but activation of the caudate portion of the brain in response to the familiar human scent showed that dogs distinguished it from all the other scents and that they had a particularly positive association with that smell. Dogs had a more positive response to familiar humans than to either unfamiliar humans or to members of their own species, whether familiar or unfamiliar.

Interestingly, the four dogs in this study who are service dogs had the strongest responses to human scents, which may be due to genetics, their intense exposure to humans during training or even simply a fluke related to small sample size. It is possible that dogs whose caudate is highly responsive to human scent may be best suited for service work. Because not all dogs selected to be service dogs end up successfully completing the time-consuming and expensive training, choosing those dogs who are most likely to succeed could save time and money as well as lessen the extensive waiting times for people in need of such dogs.